This invention relates to an energy modulatable laser unit, such as a semiconductor laser unit for use in an image recording device, such as a laser beam printer.
Generally, image recording devices, such as a laser beam printer which electrophotographically prints out character data or image data converted into dot-decomposed electrical signals, is constructed as illustrated in FIG. 3.
In FIG. 3 a laser beam emitted from a laser beam emitter 1 and modulated in accordance with image data is deflected by a polygonal scanner 2, which comprises a rotatable multi-sided mirror. The deflected laser beam scans a charged surface of a photoconductive drum 3 so as to form a coresponding latent image thereon. Toner is then electrostaticallly adhered onto the latent image at a developing station 4, and the resulating toner image is transterred onto a recording sheet 5 at a transferred station 6.
In a laser beam printer as described above, a semiconductor laser is mostly used as the laser beam emitter 1. The semiconductor laser has such characteristics as being easy to modulate the beam, as well as being small in size. Further, the semiconductor laser has recently become available at a low cost due to mass production.
In the laser beam printer employing the semiconductor laser, the semiconductor laser is controlled to be turned ON or OFF only for the formation of dots. In other words, the dots thus formed themselves carry no information regarding distinction in their brightness or darkness. Accordingly, with such dots, half-tones of original images cannot be expressed. There is a method for graded expression by either increasing or decreasing, the number of dots per unit area so as to express half tones with dots of an uniform area. However, with this method, the occurrence of moire fringes (interference fringes) inevitably arises. Thus, expressing delicate half-tones is almost impossible.
Nevertheless, in order to realize expressions of delicate half-tones, which has been regarded as impossible as described above, attempts have been made to produce half-tones such as obtaining multiple level tones by modulating the energy of laser beams to control a distribution density of electrostatic charges of the latent image form on the photoconductive element. With this control, toner density of the developed and therefore transfered image can be differentiated in its tone.
However, regarding the semiconductor laser, as indicated in FIG. 4, a threshold current for generating a laser beam varies depending upon surrounding temperatures. In addition, an individual semiconductor laser has its own property, that is, relationship between an electric current value and an output energy level the differs from one to the other. In view of the aforementioned conditions, it is extremely difficult to definitely control the energy level of laser beams through changes in electric current value.